Positron annihilation studies of defects and interfaces in ZnS nanostructures of different crystalline and morphological features

Abstract

Nanostructures of ZnS, both particles and rods, were synthesized through solvothermal processes and characterized by x-ray diffraction and high resolution transmission electron microscopy. Positron lifetime and Doppler broadening measurements were made to study the features related to the defect nanostructures present in the samples. The nanocrystalline grain surfaces and interfaces, which trapped significant fractions of positrons, gradually disappeared during grain growth, as indicated by the decreasing fraction of orthopositronium atoms. The crystal vacancies present within the grains also trapped positrons. These vacancies further agglomerated into clusters during the thermal treatment given to effect grain growth. The positron lifetime was remarkably large at extremely small grain sizes (approximately 1.5 nm) and this was attributed to the occurrence of quantum confinement effects, as verified through optical absorption measurements. Positron lifetimes in ZnS nanorods increased with increasing content of cubic phase in the samples and this observation is assigned to the annihilation of positrons in sites with increased cubic unit cell volume. The Doppler broadened spectra also indicated qualitative changes consistent with these observations.

Positron annihilation studies of NiO nanoparticles prepared through two different chemical routes

“…2). Similar observations in CdS [5] and ZnS [6] nanoparticles were earlier attributed to quantum confinement effects. The increase in the band gap, as estimated from optical absorption data (Table II), confirmed this argument.…”

Evolution of Vacancy-Type Defects, Phase Transition, and Intrinsic Ferromagnetism during Annealing of Nanocrystalline TiO2 Studied by Positron Annihilation Spectroscopy

“…When the dimension of the semiconductor materials is reduced to the nanometer length scale, the presence of structural defects and their various aspects like size, concentration, and charge states become very important parameters capable of controlling the physical properties of the nanostructures. , Furthermore, the large interfacial areas of the grains of nanostructures enriched with the free volume defects have considerable influence in tailoring their properties as well. , Therefore, for a proper understanding of the physical properties of semiconductor nanostructures, great efforts have been made to account for the presence of defects and defects-related disorders in their crystalline structure. In this context, positron annihilation spectroscopy (PAS) is the most reliable and powerful technique for probing the defects like vacancies and interfaces in semiconductor nanostructures. , There are already reports on the success of this technique in the characterization of vacancy-type defects in TiO 2 by Kong et al and Jiang et al The former work demonstrated an enhancement in the photocatalytic efficiency in samples synthesized at increasing temperatures and attributed the same to a reduction of bulk defects and improved separation of photogenerated electrons and holes. The latter report spoke on the presence of small neutral Ti 3+ -oxygen vacancy associates and large vacancy clusters in hydrogenated TiO 2 .…”

Microstructure Hierarchical Model of Competitive e+-Ps Trapping in Nanostructurized Substances: from Nanoparticle-Uniform to Nanoparticle-Biased Systems

“…The fine-grained component, the zink sulphide ZnS belonging to II–VI group compound wide band-gap semiconductors, exists in the form of hexagonal wurtzite and cubic zink blende [ 21 ]. Whichever crystal preparation technology, this material demonstrates bulk positron lifetimes τ b ranging within 0.215–0.230 ns domain [ 22 – 24 ] (in good accordance with theoretical calculations [ 25 ]), vacancy-related components (0.266 ns for monovacancy and 0.286 ns for divacancy [ 22 ]), and longer lifetime of 0.430 ns attributed to voids or grain boundaries [ 23 ].…”